/*
    This file is part of the substandard lisp implementation of Standard Lisp
    Copyright (C) 2011 Nicky Nickell

    substandard lisp is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

(dm pop (u)
  (let ((stack (second u))
         (where (third u)))
    (if (null where)
      `(prog (val)
         (setq val (car ,stack))
         (setq ,stack (cdr ,stack))
         (return val))
      `(progn (setq ,where (car ,stack))
                  (setq ,stack (cdr ,stack))
                  ,where))))

/*---
name: pop

doc: |
  explaining what pop is / how it is used!

tests: |
  (setq l '(1 2 3 4 5))
  (pop l) -> 1
  l -> (2 3 4 5)
  (pop l e) -> 2
  e -> 2
  (pop l) -> 3
  (pop l) -> 4
...*/
